Generally, a three-color separation optical system is arranged between a lens and the image pick-up planes in a commercial-grade video camera lens that uses three solid-state image pick-up elements. Therefore, a very long back focus has been desired for such a lens in order to accommodate the three-color separation optical system. Recently, solid-state image pick-up elements with a large number of picture elements have been developed in order to obtain a high picture quality. This, in turn, has caused a correspondingly high performance requirement for the camera lens. A so-called retro-focus-type lens has frequently been used to provide a long back focus lens, and such optical systems as disclosed, for example, in Japanese Laid-open Patent Applications H5-134147, and H9-292565 are known.
However, the retro-focus-type lens described in H5-134147 does not form a bright image due to the FNO being higher than 1.4. Although a bright image is achieved using the F/1.4 lens described in H9-292565, neither of these lenses is satisfactory in terms of optical performance. Thus, room for lens design improvement exists.
In the case of arranging a three-color separation optical system between a lens and an image pick-up plane, the problem of color shading must be considered. Color shading is a phenomenon that results from the incidence angles of rays onto a dichroic film being different, causing the light path lengths within the thin film to be different. This, in turn, causes the spectral characteristics of a dichroic film to vary, depending on the incidence angle. This, for example, can result in the outgoing rays from a dichroic film which contribute to the upper part versus the lower part of a picture having different colors. Thus, in a wide-angle lens that employs a three-color separation optical system, it is desirable to suppress the occurrence of color shading and to favorable correct various aberrations.
The present invention is a fixed-focus-type camera lens which may be employed in electronic cameras, etc., that use a solid-state image pick-up element. More particularly, the objects of the present invention are to provide a retro-focus-type lens which has a field angle of about 38xc2x0, an FNO as low as 1.4, so as to provide a high quality image that is especially suitable for HDTV, and which suppresses color shading and has a long back focus.